Double mastectomy offers no survival benefit to women with breast cancer

Women who are diagnosed with breast cancer in one breast, even at an early stage, sometimes opt for a double mastectomy, for fear that the cancer will migrate to the other breast.

But that decision may not offer any real survival benefit, according to a comprehensive new study of more than 600,000 patients followed over two decades.

Canadian researchers have reported that although removing an unaffected breast reduces the risk of cancer developing in that area, it does not translate into a change in the patient’s risk of dying from breast cancer.

“Prevention of contralateral (other breast cancer) cancer by preventive surgery does not appear to reduce the risk of death over the 20-year period,” concluded a team led by Dr. Steven Narod of Women’s College Hospital in Toronto. He is also a professor of obstetrics and gynecology at the University of Toronto.

His team published its findings on July 25 in the journal JAMA Oncology.

As the researchers noted, rates of bilateral (both breasts) mastectomy have been increasing for years among women with cancers detected in only one breast.

“This is despite consensus guidelines advising against the procedure in average-risk women,” they added.

Could a woman’s decision to have her second, unaffected breast removed be justified in terms of survival?

To find out, Narod’s team looked at U.S. cancer data from more than 661,000 women who were all diagnosed with cancer confined to one breast between 2000 and 2019. The cancers ranged from the earliest stage of breast cancer, called ductal carcinoma in situ (DCIS), to stage 3 invasive cancers.

The results were followed for three groups of about 36,000 women each, separated by type of treatment. One group underwent the least radical type of breast cancer surgery, called a lumpectomy; the second group chose to have only the affected breast removed (unilateral surgery); and the third group opted for a double mastectomy.

Over two decades of follow-up, far fewer women developed cancer in their second breast if they had a double mastectomy: just 97 cases out of about 36,000, compared with 766 cases among women who had a lumpectomy and 728 among women who had one breast removed.

Overall, the risk of a woman with breast cancer developing cancer later in the second breast was low, just under 7%. This means that about 69 out of 1,000 women with unilateral cancer will develop cancer in the other breast over a 20-year period.

But surprisingly, the reduction in second breast cancers in women who had a double mastectomy did not provide any benefit in terms of breast cancer survival.

Over the 20 years of the study, breast cancer deaths were similar in all groups: 3,077 (8.5%) of women in the lumpectomy group, 3,269 (9%) in the unilateral mastectomy group and 3,062 (8.5%) in the double mastectomy group, the researchers found.

If double mastectomy reduces recurrence in the other breast, why wouldn’t it increase survival?

The answer to that question may lie in why women with breast cancer die in the first place, the Toronto team says. Typically, death occurs when the cancer spreads to other parts of the body.

The new findings suggest that cancers that appear years later in a second breast are both rare and may often be new cancers, unrelated to the previous tumor. And in many cases, they are easily treated before they spread, Narod’s team explained.

They note that when cancers appeared in a second breast, they were “on average smaller” and less likely to be an aggressive form of breast tumor.

Speaking to The New York Times, Narod speculated that what most often kills women with breast cancer is not a second breast cancer, but the first tumor spreading to other parts of the body.

Overall, the findings “call into question the metastatic potential” of new cancers arising in a second breast after initial breast cancer treatment, the study authors said.

Dr. Seema Asha Khan and Dr. Masha Kocherginsky are breast cancer care experts at Northwestern University in Chicago and co-authors of a journal commentary on the new study.

They called the issue of contralateral breast cancer a “complex biological enigma.”

They noted that the Toronto study did indeed find that the minority of women who developed cancer in their second breast were four times more likely to die from the disease than women whose second breast remained healthy.

Given these results, why did double mastectomies not lead to better survival?

Khan and Kocherginsky agreed that this is a puzzle, and some aspects of the methodology used in the new study (and previous ones) might help explain the contradiction.

They also noted that many patients opt for a double mastectomy for reasons other than fear of cancer recurrence.

“There are certainly individuals who, with a good understanding of the risks and quality of life issues associated with bilateral mastectomy with or without reconstruction, would prefer to avoid both the experience of breast surveillance imaging (mammography) and the burden of undergoing treatment for a second breast cancer (even if it has a high chance of being cured),” they wrote.

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